1. Field of the Invention
This invention relates to treatment of wastewaters by the compete mix activated sludge process and particularly relates to utilization of this process in a single tank or basin for nitrification of ammonia in the wastewaters and denitrification of the nitrites and/or nitrates formed therefrom.
2. Review of the Prior Art
Complete mix systems are designed so that if samples are taken simultaneously over the basin area, the measured properties are essentially uniform. As one of these properties, the dissolved-oxygen content (D.O.) is maintained as uniformly as possible at an average dissolved-oxygen content of 2.0 mg of O.sub.2 /l. In practice, the D.O. concentration is usually not uniform because higher D.O. concentrations are found closer to the aerators and to the liquid surface (particularly if surface aerators are used) and because lower D.O. concentrations are found near the sides and the bottom of the basin.
Complete mixing is commonly conducted in round, square, or rectangular tanks or basins into which incoming wastes are fed at numerous places. The contents of the basins are sufficiently mixed to insure that the incoming wastes are rapidly dispersed throughout the basins, in contrast to plug-flow systems. The volume of mixed liquor in a basin is so much greater than the volume of the incoming wastewater that the wastewater is overwhelmingly dominated by the basin contents. Thus there is a relatively uniform food/microorganism ratio existing in such complete-mix basins. Also, there is a uniform concentration of mixed liquor-suspended solids (MLSS) to be found in complete mix aeration basins, as contrasted with the variable concentration noted in the plug-flow and semi-plug flow tanks.
It should be understood that the mixed liquor in a complete mix activated sludge basin does not flow translationally, as in a smoothly flowing river or an oxidation ditch. Instead, it moves onward very slowly, the hydraulic retention time within the basin typically being 12-60 hours, depending upon the strength of the incoming wastewater and the treatment requirements. However, it is not stagnant because the mixing devices move the liquor vertically, horizontally, and radially. A toroidal flow pattern around each mixing device is indeed a common occurrence so that each particle of mixed liquor is exposed repeatedly but randomly to contact with oxygen while the aerators are in operation.
Ammonia, derived from decomposition of proteins, is present in many wastewaters as a contaminant that must be removed. The mixture of microorganisms that exists in a barrier oxidation ditch is very well suited for such removal by ammonia oxidation to nitrite with Nitrosomas (e.g., Nitrosomas europea), oxidation of nitrite to nitrate with Nitrobacter (e.g., Nitrobacter winnogradski and Nitrobacter agilis) and denitrification by reduction of the nitrite and/or nitrate to nitrogen gas with facultative heterotrophic microorganisms generally of the genera of Pseudomonas, Achromobacter, Bacillus, and Micrococcus. All of these microorganisms are ubiquitous in the environment. Both Nitrosomas and Nitrobacter require a dissolved oxygen level in excess of approximately 0.5 mg/l and preferably greater than 1.0 mg/l.
A barrier oxidation ditch of such nitrification/denitrification capability operates on approximately a 6-18 minute cycle and contains microorganisms having a long sludge age or mean cell residence time (MCRT).
It would be highly desirable to be able to utilize such short cycle times in a complete mix activated sludge system (CMAS System). However, it is contrary to established practice to provide closely timed, brief periods of aerated mixing and nonaerated mixing in order to emulate an oxidation ditch. In consequence, most attempts to accomplish cyclical oxic-anoxic CMAS basin operation and resulting nitrification-denitrification are believed to have utilized fill-and-draw sequence batch reactors.
Because the autotrophic microorganisms such as Nitrosomas and Nitrobacter grow much more slowly, for example, on the order of five to ten times more slowly, than the facultative heterotrophic microorganisms, an acclimation period of up to one to three months may be necessary, although maintaining a pH and temperature just below the maximum and a D.O. level just above the minimum can minimize this period.
As an example of processes adapted to cope with such differences in bacterial growth rates, the nitrification process disclosed in U. S. Pat. No. 4,705,633 increases the efficiency of nitrification by increasing the population of nitrifying bacteria beyond that which would naturally occur in a nitrifying activated sludge system by using a return sludge reaeration zone which is enriched with anhydrous ammonia or an aqueous solution thereof.
The process of U.S. Pat. No. 4,537,682 controls the microorganism population by controlling the sludge wastage rate, hydraulic residence time, dissolved oxygen level, sludge mixing rate, biological oxygen demand, pH, and temperature for high-strength ammonia-containing wastewaters, possibly containing other contaminants such as phenolic, cyanide, and thiocyanide compounds, in order to nitrify and denitrify in a single reactor. Although it is true that this process is directed to the unusually difficult problem of treating high-strength industrial wastewaters, its seven areas of testing and control impose an onerous burden on a plant operator. Simpler methods of control, particularly for sanitary wastewaters and for wastewaters from food processing plants, are accordingly needed.
Other denitrifying methods also seek to remove phosphorus, as exemplified by U.S. Pat. No. 4,655,925 which discloses a method of removing nitrogen and phosphorus from wastewater by using a mixed liquor comprising the wastewater and activated sludge within a single basin in which are aerating and mixing devices which cannot independently maintain a fixed mixing rate while selectively varying the oxygen transfer rate. This limitation occurs because the aerating and mixing devices described in U.S. Pat. No. 4,655,925 use jet aeration units which provide adequate mixing and oxygen transfer during aerobic cycles but which lose mixing effectiveness during anoxic and anaerobic cycles in which the jet header unit pumps continue to be operated while the air flow to the jet pumps is discontinued. Under these conditions, it is well known that jet pumps do not mix as well as when they operate with both pumps and compressed air supply running.
An even more important consequence is that turning off air to the jets inherently decreases the mixing rate and decreases the mixing brake horsepower per unit volume of the basin. What is accordingly needed is a mixing device that has independent control of both aeration and mixing, provides fully effective mixing, and does not lose its mixing power input when aeration input is changed.
In the method of U.S. Pat. No. 4,655,925, a cycle consisting of an agitating step and an aerating step is repeated at least two times, and each cycle is finished within two hours, with the ratio of the agitating time to the aerating time being between one to one and five to one. In the agitating step, the mixed liquor is in an anoxic condition and in a succeeding anaerobic condition. Denitrification occurs in the anoxic condition, and the aerobic bacteria release phosphorus in the anaerobic condition. During the aerating step, nitrification occurs while the bacteria excessively ingest phosphorus from the mixed liquor.
However, in attempting to accomplish biological phosphorus removal by a process that includes an anaerobic cycle in which the microorganisms release phosphates to the wastewater and an aerobic cycle in which there is luxury uptake of phosphorus, it is extremely important not to have excessive sludge age in order to prevent cell breakdown and phosphorus release back to the wastewater liquid, resulting in increased effluent phosphorus concentrations. If a long sludge age is allowed to occur in the reactor, then more endogenous respiration will also occur, resulting in cell breakdown and release of stored phosphorus into the wastewater liquid so that there is increased effluent phosphorus concentration.
The cyclical activated sludge process disclosed in U.S. Pat. No. 4,655,925 must therefore be limited to short Mean Cell Residence Times (MCRT's) in order to accomplish high efficiency biological phosphorus removal. If high efficiency biological phosphorus removal is attempted through very accurate control of MCRT but at relatively low levels of MCRT in order to avoid difficulties with endogenous respiration and cell breakdown, it can become very difficult to attain a high enough MCRT to accomplish biological denitrification during the winter season. A cyclical CMAS process for a single basin that relates to nitrification and denitrification only is accordingly needed.